A marginal oscillator is a self-oscillating oscillator that generates oscillations in a tank circuit comprising inductive and capacitive elements at the natural frequency of the tank circuit. Oscillation of the tank circuit is driven by a non-linear drive circuit that sustains the oscillation on the basis of the signal across the tank circuit. The non-linear drive circuit has a non-linear gain so as to achieve a steady-state oscillation.
A marginal oscillator may be used as a sensor system to perform sensing by using elements of the tank circuit as an inductive or capacitive probe that generates an electromagnetic field in a sensing region. A characteristic of the oscillation of the tank circuit that is dependent on the electromagnetic properties of the contents of the sensing region may be detected by a detection circuit to derive a signal representing the characteristic. This, this signal is also representing the electromagnetic properties of the contents of the sensing region.
A type of marginal oscillator is a Robinson marginal oscillator, in which the non-linear drive circuit includes a gain stage that amplifies the signal across the tank circuit supplied back to the non-linear drive circuit and a limiter stage that limits the output of the gain stage for generating a drive signal supplied to the tank circuit. By separating the gain and limiter stages, the signal-to-noise ratio is improved.
Marginal oscillators were originally developed for detection of nuclear magnetic resonance. More recently, they have been used as sensor systems for sensing the electromagnetic properties of the contents of the sensing region, for example an object or a material in the sensing region. By way of example, WO-01/69168 discloses use of a Robinson marginal oscillator in a sensor system for sensing the electric or magnetic susceptibility of an object, for example to detect the position of the object. Marginal oscillators, and Robinson marginal oscillator in particular, provide particular advantages compared to other sensors, for example providing accurate sensing applicable to a wide range of systems to be sensed.